Design of Steel and Composite-Structures for Seismic Loading – Safety Requirements, Concepts and...
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Design of Steel and Composite- Structures for Seismic Loading – Safety Requirements, Concepts and Methods – Prof. Dr.-Ing. Ekkehard Fehling, University Kassel Dr.-Ing. Benno Hoffmeister, University / RWTH Aachen
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Transcript of Design of Steel and Composite-Structures for Seismic Loading – Safety Requirements, Concepts and...
Design of Steel and Composite-Structures for Seismic Loading
– Safety Requirements, Concepts and Methods –
Prof. Dr.-Ing. Ekkehard Fehling, University Kassel
Dr.-Ing. Benno Hoffmeister, University / RWTH Aachen
Design of Buildings for Seismic Actionreduced regularity
different structural systems for lateral bracing
Diagonal-verband
Diagonal-verband
Rahmen-tragwerk
discontinuous bracing systems
Diagonal bracing
frame structure
Diagonal bracing
Design of Steel Structures for Seismic ActionDuctility
Sudden or brittle failure shall not occur Examples:
Buckling Connection failureLoad
Deformation
Design of Steel Structures for Seismic ActionDuctility
Examples:
Design of Steel Structures for Seismic ActionDuctility
Specially endangered: Corner Columns
most endangered column
Design of Steel Structures for Seismic ActionDuctility
Examples:
Design of Steel Structures for Seismic ActionDissipative Behaviour
Cyclic defomability with dissipation of energy Exploitation of plastic material behaviour Principle:
Elasticbehaviour
Load
Deformation
Design of Steel Structures for Seismic ActionDissipative Behaviour
Load
Deformation
PlastificationPlastification
Cyclic defomability with dissipation of energy Exploitation of plastic material behaviour Principle:
Design of Steel Structures for Seismic ActionDissipative Behaviour
PlastificationLoad
Deformation
Plastification
Plastification
dissipatedenergy
Cyclic defomability with dissipation of energy Exploitation of plastic material behaviour Principle:
MN
Q
Design of Steel Structures for Seismic ActionDissipative Mechanisms
Bending (Frame) Normal Force (Bracings) Shear (ecc. Bracings)
Design of Steel Structures for Seismic ActionDissipative Mechanisms
Bending (Frame) Normal Force (Bracings) Shear (ecc. Bracings)
Design of Steel Structures for Seismic ActionDissipative Behaviour – Global System
Successive Formation of Plastic HInges
Load
Deformation
Design of Steel Structures for Seismic ActionDissipative Behaviour – Global System
Succesive Formation of Plastic Hinges
Deformation
Load
Design of Steel Structures for Seismic ActionDissipative Behaviour – Global System
Succesive Formation of Plastic Hinges
Deformation
Load
Design of Steel Structures for Seismic ActionDissipative Behaviour – Global System
Succesive Formation of Plastic Hinges
Deformation
Load
Design of Steel Structures for Seismic ActionDissipative Behaviour – cyclic
Experimental Investigations on Frame Structures
F - ID
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ID [%]
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ID [%]
F [k
N]
Design of Steel Structures for Seismic ActionFunctioning dissipative Mechanisms
Design of Steel Structures for Seismic ActionInadequate Dissipation Capacity
Design of dissipative Members„Overstrength“ of Material
Example S 235, nominal Yield Strength fy,k = 235 N/mm²
Stress
Strain
235Overstrength
Consequences:
in the dissipative member the forces will become bigger than intended Failure of connections
(e.g. bolts) Stability failure
(e.g. columns)
Consequences:
in the dissipative member the forces will become bigger than intended Failure of connections
(e.g. bolts) Stability failure
(e.g. columns)
Design of dissipative Members„Overstrength“ of Material
how to ensure dissipative behaviour
Stress
Strain
235Overstrength
Measures:– Capacity Design (design
of critical members and connections with „overstrength“)
– Limitation of maximum yield strength in dissipative Members
– Control of execution(strength as ordered = delivered strength?)
Measures:– Capacity Design (design
of critical members and connections with „overstrength“)
– Limitation of maximum yield strength in dissipative Members
– Control of execution(strength as ordered = delivered strength?)
Design of dissipative MembersPlastic Fatigue of Materials
Elastic Fatigue Strength Plastic Fatigue(Low Cycle Fatigue)
Design of dissipative MembersPlastische Ermüdung des Werkstoffs
Elastic Fatigue Strength Plastic Fatigue(Low Cycle Fatigue)
Δσ
104 5·106
>108
N1 100
N
ΔRpl
RplRpl
Design of dissipative MembersToughness of Material
Toughness of material – basic requirement for dissipation
Design of dissipative MembersZähigkeit des Werkstoffs
Mesures:– Selection of material
quality / grade (sufficient toughness even for low temperatures)
– Dissipative zones outside the heat influence zones due to welding
Mesures:– Selection of material
quality / grade (sufficient toughness even for low temperatures)
– Dissipative zones outside the heat influence zones due to welding
Toughness of material – basic requirement for dissipation
Design of dissipative MembersStability of cross sections
Slender cross section show premature local buckling:– dissipation will be less – premature damage
Design of dissipative MembersStability of cross sections
Measures:– Compact Cross Sections
(Cross sectional class 1)– For thin walled Structures
design for elastic behaviour consider stability aspects(e.g. fluid tanks)
Measures:– Compact Cross Sections
(Cross sectional class 1)– For thin walled Structures
design for elastic behaviour consider stability aspects(e.g. fluid tanks)
Slender cross section show premature local buckling:– dissipation will be less – premature damage
Design für Dissipative BehaviourGlobal capacity design
Npl
N
NN
V V
pl
StützeAnker
Anker Anker
g+q
NcolumnNanchor
VanchorVanchor
Design für Dissipative Behaviourlocal capacity design
Npl
Schweißnaht
Netto-querschnitt
Schrauben
Lochleibung
Measures:– avoid premature
brittle failureof non dissipative connections
for bolted / or welded connections: design with overstrength
for bolted connections: bearing stresses should be more critical than shear in bolt
Measures:– avoid premature
brittle failureof non dissipative connections
for bolted / or welded connections: design with overstrength
for bolted connections: bearing stresses should be more critical than shear in bolt
weld
net-section
Bolts
Bearing resistance
Seismic Design of Steel Structures
Codes:– EN 1998 (or: DIN 4149 = EN 1998 simplified)– codes for steel structures and materials
Seismic Design:– Make use of dissipation, assuming behaviour factor q
(Reduction of „elastic“ action)– Application of capacity design
e.g. for bolted connections:
Rbolt > Rbearing > Rcross-section,pl > Eseismic/q
for comparison: static design verification:(Rbolt , Rbearing , Rcross-section) > Ed
Flow chart for design (1)
Erster Bauwerksentwurf (z.B. für Windlasten)Ergebnis: Abmessungen, Topologie, ständige und veränderliche Lasten
Entscheidung über mögliche Dissipationsmechanismen
Lastkombination für Erdbeben
Berechnung mit elastischem AntwortspektrumVergleich der Beanspruchungen aus Wind und Erdbeben
Wind > ErdbebenJA keine weiteren
Nachweise
Ausnutzung < 150%
NEIN
JADuktilitätsklasse 1
NEIN
Duktilitätsklasse 2 oder 3 (qerf > 1,5)
Wahl des Verhaltensbeiwertsq = max. Ausnutzung [%] / 100
möglicheVerhaltensbeiwerte(Systemtopologie,Regelmäßigkeit)
Natural Ductilityq = 1,5
Preliminary design of building (e.g. for wind loads)Result: dimensions, topology, permanent and variable loads
Decision about conceivable dissipation mechanisms
Combination of actions for earthquake
Calculation using response spectrumComparison of actions due to wind and earthquake
Wind > EarthquakeNo further checks
ductility class L
Exploitation < 150 %
Possible behaviour factors (system topology,regularity)
Ductility class M or H (q >1,5)
Selection of behaviour factor q = max. exploitation [%] / 100
yes
yes
no
no
Flow chart for design (2)
Wahl des Verhaltensbeiwertsq = max. Ausnutzung [%] / 100
Berechnung mit reduziertem AntwortspektrumEd = Eelast / q
Überprüfung der Ausnutzungen (dissipative Bauteile)i.d.R. max. Ausnutzung ~ 100 %
min. Ausnutzung ~ 80%Inverser Ausnutzungsgrad = 1 / 0,80 = 1,25
Globale Kapazitätsbemessung mitg + p und 1,2 Ed
lokale Kapazitätsbemessung (Anschlüsse dissipativer Bauteile) mit1,2 Rk,plast
Schnittgrößen
Selection of behaviour factor q = max. exploitation [%] / 100
Calculation using design spectrum Ed = Eelast / q
Check of degree of exploitation (dissipative members) usually max. exploitation ≈ 100 %
min. exploitation ≈ 80 %Inverse degree of exploitation Ω = 1 / 0,80 = 1,25
global capacity design with g + q and 1,2 Ω Ed
local capacity design (connection of dissipative elements)
member forces
Application Example:
Reactor- and Heater Towersfor a steel producing direct reduction plant
in Indonesia
Assuming an Elastic system
atop = 0,5 … 1,0 g
ag = 0,2 … 0,4 g
Ground and Response Acceleration
atop
1 g horizontal = ......
Assuming an Elastic system
Ductility: where to get it from?
not o.k. ! not o.k. !
buckling = failure
Ductility: where to get it from?
o.k. ! o.k. !
Buckling o.k.
First possible solution
Dissipative Elements
Example: Shear –Link in Eccentrically Braced Frame (EBF)
Vpl
V
Second possible solution Vertical Shear links
Design of Shear Links
Biggest possible ductility in shear Avoid flexural failure mode Web buckling should occur at large
deformations only Ensure lateral stability of flanges
Capacity Design: 2nd loop of calculation
from shear link: Vpl
Calculate system again with
Vpl * γRd !Design columns, beams
and diagonals for this load
Vpl * γRd
Spacing of stiffener plates, type of link Plastic deformability θ= 0.02 .. 0.08 rad
Conclusions
Design for Earthquake requires different way of thinking: verification of behaviour rather than verification of strength
The behaviour of a structure under seismic loading is mainly determined by:– Regularity – avoid extreme straining/ loading of certain members– Redundancy – enable reserves of saftey– Ductility – plastic deformations without premature failure– Dissipation – from formation of cyclic plastic hystereses– Quality and Control of Execution – too much of strength
may be dangerous
